A specific geometric stimulus pattern (a wheel) can resonate with oscillations and induce an illusory perception of flicker (Sokoliuk & VanRullen, Journal of Neuroscience, 2013). Conversely, a steadily flickering light is known to produce visual hallucinations of geometric patterns; the exact perceived pattern depends on the stimulation frequency. Presumably, the flicker entrains neuronal oscillations that in turn give rise to the geometric hallucination. In order to shed light on this interaction, we first mapped the relationship between the temporal frequency of flicker stimulation and the geometrical organization of illusory patterns reported by subjects. Then we reversed this relation, and measured the effects of actually viewing specific geometric shapes on ongoing EEG activity. 8 subjects observed a homogeneous visual field flickering at different frequencies from 3 to 40 Hz. At the end of each 100-s trial, they were asked to describe any hallucinatory pattern(s) experienced, and indicate its perceived vividness. The most reported shapes were wheels and spirals. Wheels were significantly more likely to occur (and more vivid) below 10Hz, whereas spiral hallucinations peaked between 10 and 20Hz. Then, based on these subjectsâ€™ descriptions, we created prototypical pictures of a wheel and a spiral illusory percepts, and equalized their contrast and 2D Fourier power spectra. We presented the two static pictures (in randomly interleaved 12-s trials) to a distinct group of 11 subjects undergoing EEG recording. The EEG power spectra associated with the two patterns showed significant differences over occipital and parietal electrodes. Perception of the wheel evoked higher activity at EEG frequencies below 10Hz, while the spiral evoked significantly higher EEG activity between 10 and 20Hz. We conclude that the link between neuronal oscillations and geometric patterns is bidirectional: flicker stimulation induces frequency-specific geometric hallucinations, and the same statically presented geometric shapes selectively enhance the same brain oscillatory frequencies.